1. Field of the Invention
The present invention relates to a biscarbazole derivative, a material for an organic electroluminescence device, and an organic electroluminescence device using those.
2. Description of Related Art
A known organic electroluminescence device includes an organic thin-film layer between an anode and a cathode, the organic thin-film layer including an emitting layer, and emits light using exciton energy generated by a recombination of holes and electrons injected into the emitting layer (see Patent Literature 1: WO2003-080760, Patent Literature 2: Japanese Patent No.: 3139321, Patent Literature 3: Japanese Patent No.: 4357781, Patent Literature 4: JP-A-2003-151774, Patent Literature 5: JP-A-2008-135498, Patent Literature 6: JP-A-2009-21336, Patent Literature 7: JP-A-2008-214307).
Such an organic EL device, which has the advantages as a self-emitting device, is expected to serve as an emitting device excellent in luminous efficiency, image quality, power consumption and thin design.
In forming the emitting layer, a doping method, according to which an emitting material (dopant) is doped to a host, has been known as a usable method.
The emitting layer formed by the doping method can efficiently generate excitons from electric charges injected into the host. With the exciton energy generated by the excitons being transferred to the dopant, the dopant can emit light with high efficiency.
Recently, in order to improve performance of the organic electroluminescence device (hereinafter, occasionally referred to as an organic EL device), a doping method has been further studied to find a suitable host material.
Such a host material is disclosed in, for instance, Patent Literatures 1 to 7. Patent Literatures 1 to 7 disclose a compound including a carbazole skeleton and a nitrogen-containing aromatic ring in the same molecule and a compound including a plurality of carbazole skeletons in the same molecule, as shown in the following compounds I to VIII.
The compounds I and II disclosed in Patent Literature 1 each have a structure in which a carbazole skeleton is bonded to a benzene ring and an electron-deficient nitrogen-containing hetero aromatic ring structure. A carbazole skeleton, which is represented by polyvinyl carbazole, has been known as a main skeleton of a hole transporting material. In contrast, the electron-deficient nitrogen-containing hetero aromatic ring structure has been known as a structure having a high electron transporting capability. In other words, the compounds I and II disclosed in Patent Literature 1 are materials for balancing charge transportation by combining a hole transporting skeleton and an electron transporting skeleton.

The compound I has only a single carbazole skeleton and lacks a hole transporting capability, so that a favorable luminescence property cannot be obtained. The compound II has two carbazolyl groups that are branched to left and right relative to a bond axis of a pyrimidine ring and a benzene ring (two conjugated aromatic ring). Accordingly, an overlapping margin of the carbazole skeleton between molecules is impaired, so that a hole transporting capability is insufficient and a re-bonding position of charges is likely to be closer to the anode. Consequently, favorable luminescence property and lifetime property cannot be obtained.
In order to enlarge the overlapping margin between the molecules and exhibit a sufficient hole transporting capability, it has been proposed to incorporate a structure in which carbazole skeletons are linked in the molecules. For instance, the compounds III to VI disclosed in Patent Literatures 2 to 5 have a structure in which two carbazole skeletons are linked. However, since none of the compounds III to VI has an electron-deficient nitrogen-containing hetero aromatic ring structure, adjustment of carrier balance between holes and electrons is difficult, so that a favorable luminescence property cannot be obtained.
The compound VII disclosed in Patent Literature 6 has an electron-deficient nitrogen-containing hetero aromatic ring structure and a carbazole-linking structure. However, two carbazole skeletons are bonded to a carbon atom at 3-position by a nitrogen atom. In this structure, the two carbazole skeletons are twisted to each other to lose flatness. Accordingly, the overlapping margin between the molecules becomes small and a hole transporting capability becomes insufficient, so that favorable luminescence property and lifetime property cannot be obtained.
The compound VIII disclosed in Patent Literature 7 has a structure in which a bipyridyl group (a nitrogen-containing aromatic heterocyclic group) is bonded to a benzene ring of a carbazole skeleton. The compound is not disclosed as a phosphorescent host material although being used as a material for an electron transporting layer. However, since the compound is considered to exhibit a high electron transporting capability, when used as a host material, the compound provides a poor carrier balance within the emitting layer and fails to exhibit a favorable luminescence property.